Normal stresses, non‐Newtonian flow, and dynamic mechanical behavior of polymer solutions

Abstract

AbstractComplex dynamic shear moduli, shear and normal stresses in steady shearing flow, and complex moduli with superposed steady shear are investigated in both theoretical and experimental aspects. Firs) it summary of phenomenological and molecular theories is given. Experimental tests of phenomenological theory are made by comparing dynamic (linear viscoelastic) data and steady shearing flow (nonlinear viscoelastic) data. It is found that the recent theory of memory fluids is quite satisfactory and also the results show that the action of normal stress in steady shearing flow is mainly pull in the direction of the flow line. The behavior of dilute polymer solutions (near A condition are best approximated by the molecular theory of Zimm, particularly at very low shear rates. At higher shear rates, the Zimm theory fails in predicting the appearance of non‐Newtonian viscosities and nonquadratic normal stresses. Various proposed modifications of the theory are examined experimentally. The behavior of concentrated polymer solution does not seem to fit any of the existing molecular theories; therefore, some typical features of the experimentally observed behavior of concentrated solutions are simply summarized. The results of the measurements of complex moduli with superposed steady shear show that in the low‐frequency region the effect of superposed shear rate results in a much larger decrease in the real part of the complex modulus that in the imaginary part, while in the higher‐frequency region the real part is again decreased but the imaginary part is increased with the increase of superposed shear rate.